1. Field of the Invention:
This invention relates to a method of producing ferro-nickel or metallic nickel, and more particularly to a method of producing ferro-nickel or metallic nickel from iron-rich nickel ore or iron-rich nickel oxide by adding separately prepared silica to the starting material.
2. Description of the Prior Art
Nickel is an indispensable metal for making stainless steel and heat-resisting alloy. As the consumption of nickel increases, the availability of high grade nickel ores gradually decreases, and there has been an increasing need for a process of making nickel from comparatively low grade ores, such as laterite, containing a large amount of iron, as shown in Table 1. There have been a number of studies made heretofore on the process of producing ferro-nickel from iron-rich nickel ore. Among such studies, the nickel-preferred reduction is most important.
What is meant by the "nickel-preferred reduction" is based on the difference in the thermodynamic properties of nickel and iron; namely, iron has a greater affinity for oxygen than has nickel. In an actual refining process for the reduction of iron-rich nickel ore, such as laterite, however, iron and nickel are simultaneously reduced, and it is very difficult to give preference to the reduction of nickel over that of iron. More particularly, when iron-rich nickel ore is reduced in a conditioned gaseous atmosphere, both nickel and iron are simultaneously formed by reduction, and the resultant ferro-nickel has a comparatively low nickel content. It has been difficult to improve the nickel content in the ferro-nickel by any further treatment.
The inventors have noticed the fact that iron contained in iron-rich nickel ores can be fixed in the form of difficultly reducible fayalite, so that the selective reduction of nickel can be effected based on the difference of the reducibility between fayalite and nickel compound.
The formation and decomposition of fayalite (2FeO.SiO.sub. 2) can be represented by the following chemical formulae.
2Fe.sub.3 O.sub.4 + 3SiO.sub.2 + 2CO .fwdarw. 3(2FeO.SiO.sub. 2) + 2CO.sub.2 ( 1) PA1 2feO.SiO.sub. 2 + 2CO .fwdarw. 2Fe + SiO.sub.2 + 2CO.sub.2 ( 2)
The formation of fayalite according to the formula (1) takes place already in an atmosphere with carbon-monoxide content ##EQU2## of 1.6% or more, while the decomposition of fayalite according to the formula (2 when the carbon-monoxide content is up to 90%. Accordingly, if the iron component of the nickel ore is fixed in the form of fayalite by reaction formula (1), the reduction of nickel can be selectively effected while suppressing the reduction of iron.
Therefore, an object of the present invention is to provide a method for producing ferro-nickel or metallic nickel from iron-rich nickel ores by adding silica so as to positively use the formation lof fayalite for the selective reduction of nickel.
U.S. Pat. No. 2,767,075, which was granted to Albert E. Greene on Oct. 16, 1956, discloses a process of direct reduction of an iron ore containing nickel, in which it was intended to hold oxide of iron as an iron oxide silicate against reduction. To this end, Greene uses fluxing material which is to slag oxide material including oxide of iron. Greene, however, does not suggest the effective use of fayalite, due to the following reasons.
a. Greene uses a molten metal bath on which a starting charge is placed. As a result, the starting charge including the nickel-containing material and silica-containing fluxing material is susceptible to melting. Once the starting charge is molten, the chemical reaction of the aforesaid equation (1) will not take place. It is generally believed that, when melted, fayalite is ionized, as given by the equation of 2Fe.SiO.sub. 2 .fwdarw. 2Fe.sup.+.sup.+ + O.sup.-.sup.- + SiO.sub.2. As soon as being ionized, the desired difficultly reducible nature of the slag is lost, and the efficiency of the selective reduction is greatly deteriorated.
b. The inventors of the present application have found that the formation of the fayalite is carried only in solid state, and the reaction of the fayalite formation is very low. The fact of solid state reaction may be one of the reasons for the slowness of the fayalite formation. With the arrangement of Greene, the starting charge may easily be intermingled with the molten bath, so that the ingredients of the starting charge cannot be held in the solid state for a sufficiently long period of time for effecting the selective reduction.
c. Greene succeeded in producing "a direct nickel-iron containing approximately 6% of nickel" (column 2, lines 52-53). Such nickel content in the nickel iron is unduly low for any practical processes which effectively utilize fayalite. More particularly, the so-called Strategic-Udy process based on the selective use of carboneous reductants without any fayalite can produce a ferro-nickel containing 21.22% of nickel, as disclosed by Marvin J. Udy and Murray C. Udy in Journal of Metals, May 1959 page 312. In other words, when the starting charge of Greene was placed on the molten bath, the reduction of the starting ore was mostly carried out by the so-called "selective reduction with controlled carboneous reductants", rather than by utilizing the presence of fayalite.
d. Greene suggests the use of an electric furnace for carrying out his process. It is not necessarily economical to heat the starting charge by the comparatively expensive electric energy. For instance, iron ores and nickel ores as delivered usually have a large amount of water of crystallization and moisture attached thereto. Removal of the water of crystallization and moisture by electric energy may become unreasonably costly.
To overcome the aforesaid difficulties of conventional methods, the inventors have carried out a series of studies and tests. As a result, a new method of reducing iron-rich starting material containing nickel is provided.